1. Steady-state fluorescence polarization has been used to detect segmental motions in proteins. Heating of glycerol solutions reveals thermally-activated rotations, while global rotations are obtained in isothermal experiments. The effective size of the rotating unit decreases with temperature. 2. Serum albumin acts as an esterase with certain fluorogenic compounds including acetylsalicylic acid, or aspirin. The rates of reaction have been studied by manual and stopped-flow fluorescence. The results show an unsually reactive site on albumin, which serves as a model for other protein acetylations. 3. Phosphorescence and delayed fluorescence studies of biphenyl and naphthalene in fozen cyclohexane at carbon dioxide temperatures show that diffusion occurs in this solid yet plastic matrix.